Polyolefin resins are hitherto available on the marketplace in the form of powder or pellets. When a polyolefin resin in such a state is used to form a film on a surface of a substrate by coating or the like method, it is necessary to use a thermoforming machine such as an extruder or a laminator. Thus, the polyolefin resin when coated in a melted state must be melted and kneaded. This not only brings about a need for a great energy but also causes a problem that the physical properties of the polyolefin resin are affected as a result of the kneading.
In this circumstance, a coating method in which a coating liquid containing a polyolefin resin dissolved in an organic solvent is used has been considered. When this method is performed using a coating liquid having a higher polyolefin resin concentration, however, the coatability thereof is deteriorated and the film thickness is apt to be non-uniform due to an increase of the viscosity thereof. When the polyolefin resin concentration is reduced, on the other hand, there arises a problem that the coating operation must be repeated a number of times in order to obtain a desired film thickness.
As a method to solve the above problems, a thought may occur to use a dispersed liquid, obtained by dispersing a polyolefin resin in water in the form of fine particulates, for coating. By applying and drying such a dispersed liquid onto a surface of a plastic, paper, metal or the like substrate using a generally employed coater, printer or sprayer, it is possible to impart water resistance, chemical resistance and oil resistance to the substrate. Further, the dispersed liquid may be used as a heat sealing material. Thus, using a polyolefin resin in the form of a dispersed liquid, it is possible to easily form a polyolefin resin film over a surface of a substrate.
It is, however, not easy to disperse fine particulates of a polyolefin resin in water. For example, an aqueous emulsion containing a polyolefin derivative, an emulsifier and a water-insoluble organic solvent and a method for producing the aqueous emulsion are disclosed (see, for example, Patent Document 1). As the polyolefin derivative, specifically used is a hydrogen-added derivative of a hydroxyl group-containing conjugated diene polymer, though a polypropylene is exemplified. Since such a hydrogen-added derivative is produced by a two-stage process including polymerization of a diene compound followed by hydrogen-addition of the obtained polymer, the productivity thereof cannot be said to be high. Further, in order to solidify a coated film, it is necessary to add a curing agent to the aqueous emulsion and to heat the coated film for curing same.
A technique in which a polypropylene wax is used in lieu of the above-described hydroxyl group-containing conjugated diene polymer (see, for example, Patent Document 2). In particular, Patent Document 2 discloses an aqueous polyolefin wax dispersion liquid containing a polypropylene wax having a specific average particle diameter and a specific acid value, and a specific glyceride of a specific unsaturated monocarboxylic acid. This technique, however, has a problem with respect to the smoothness of the coated film because the polypropylene wax is present as a solid in the dispersion.
Incidentally, a low crystalline polyolefin (see, for example, Patent Documents 3 and 4) may be used for surface modification of a highly crystalline polypropylene film. When the low crystalline polyolefin is used for surface modification of a highly crystalline polypropylene film, however, it is necessary to dissolve a polymer of the low crystalline polyolefin in an organic solvent. Thus, there is an increasing demand for an aqueous paint which does not adversely affect the environment and which is harmless to the operators.
[Patent Document 1]
Japanese Patent Application Publication No. 2003-261724
[Patent Document 2]
Published Japanese Translation of PCT Application No. H09-502216
[Patent Document 3]
Japanese Patent Application Publication No. 2001-172325
[Patent Document 4]
Japanese Patent Application Publication No. 2001-172325